Simple pure CSS dropdown menu by Phil Hoyt

<h1>Simple Pure CSS Drop Down Menu</h1>

<!-- Use nav tag as a wrapper for the dropdown menu -->
<nav id="primary_nav_wrap">
  <!-- Use ul tag as a list of menu items -->
  <ul>
    <!-- 
	  + Use li tag as a menu item follow conditions:
	  	- If a menu item doesn't have submenu items, 
		  just using anchor tag with a "href" and a text content.
	  	- Otherwise, if a menu item has submenu items, 
		  using ul tag again to make nested lists.
	  
	  + Next, if a menu item is active, 
	  	set class with value "current-menu-item" for it.
	-->
    <li class="current-menu-item"><a href="#">Home</a></li>
    <li>
      <a href="#">Menu 1</a>
      <ul>
        <li><a href="#">Sub Menu 1</a></li>
        <li><a href="#">Sub Menu 2</a></li>
        <li><a href="#">Sub Menu 3</a></li>
        <li>
          <a href="#">Sub Menu 4</a>
          <ul>
            <li>
              <a href="#">Deep Menu 1</a>
              <ul>
                <li><a href="#">Sub Deep 1</a></li>
                <li><a href="#">Sub Deep 2</a></li>
                <li><a href="#">Sub Deep 3</a></li>
                <li><a href="#">Sub Deep 4</a></li>
              </ul>
            </li>
            <li><a href="#">Deep Menu 2</a></li>
          </ul>
        </li>
        <li><a href="#">Sub Menu 5</a></li>
      </ul>
    </li>
    <li>
      <a href="#">Menu 2</a>
      <ul>
        <li><a href="#">Sub Menu 1</a></li>
        <li><a href="#">Sub Menu 2</a></li>
        <li><a href="#">Sub Menu 3</a></li>
      </ul>
    </li>
    <li>
      <a href="#">Menu 3</a>
      <ul>
        <li class="dir"><a href="#">Sub Menu 1</a></li>
        <li class="dir">
          <a href="#"
            >Sub Menu 2 THIS IS SO LONG IT MIGHT CAUSE AN ISSEUE BUT MAYBE
            NOT?</a
          >
          <ul>
            <li><a href="#">Category 1</a></li>
            <li><a href="#">Category 2</a></li>
            <li><a href="#">Category 3</a></li>
            <li><a href="#">Category 4</a></li>
            <li><a href="#">Category 5</a></li>
          </ul>
        </li>
        <li><a href="#">Sub Menu 3</a></li>
        <li><a href="#">Sub Menu 4</a></li>
        <li><a href="#">Sub Menu 5</a></li>
      </ul>
    </li>
    <li><a href="#">Menu 4</a></li>
    <li><a href="#">Menu 5</a></li>
    <li><a href="#">Menu 6</a></li>
    <li><a href="#">Contact Us</a></li>
  </ul>
</nav>

#primary_nav_wrap {
  margin-top: 15px;
}

#primary_nav_wrap ul {
  list-style: none; /* default value of list-style is disc */
  position: relative;
  float: left;
  margin: 0;
  padding: 0;
}

#primary_nav_wrap ul a {
  display: block; /* the default value is inline-block */
  color: #333;
  text-decoration: none;
  font-weight: 700;
  font-size: 12px;
  line-height: 32px;
  padding: 0 15px;
  font-family: 'HelveticaNeue', 'Helvetica Neue', Helvetica, Arial, sans-serif;
}

#primary_nav_wrap ul li {
  position: relative;
  float: left;
  margin: 0;
  padding: 0;
}

#primary_nav_wrap ul li.current-menu-item {
  background: #ddd; /* change background color of the active menu item */
}

#primary_nav_wrap ul li:hover {
  background: #f6f6f6; /* Change color of a menu item when user hovers it */
}

#primary_nav_wrap ul ul {
  display: none; /* Normally, hide all nested list */
  position: absolute;
  top: 100%;
  left: 0;
  background: #fff;
  padding: 0;
}

#primary_nav_wrap ul ul li {
  float: none;
  width: 200px;
}

#primary_nav_wrap ul ul a {
  line-height: 120%;
  padding: 10px 15px;
}

#primary_nav_wrap ul ul ul {
  top: 0;
  left: 100%;
}

#primary_nav_wrap ul li:hover > ul {
  display: block; /* when user hovers a menu item, then show its nested list */
}